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United States Patent |
6,191,429
|
Suwa
|
February 20, 2001
|
Projection exposure apparatus and method with workpiece area detection
Abstract
Improvements in a focusing apparatus having an objective optical system for
optically manufacturing a workpiece, forming a desired pattern on a
surface of a workpiece or inspecting a pattern on a workpiece and used to
adjust the state of focusing between the surface of the workpiece and the
objective optical system. The focusing apparatus has a first detection
system having a detection area at a first position located outside the
field of the objective optical system, a second detection system having a
detection area at a second position located outside the field of the
objective optical system and spaced apart from the first position, and a
third detection system having a detection area at a third position located
outside the field of the objective optical system and spaced apart from
each of the first and second positions. A calculator calculates a
deviation between a first focus position and a target focus position and
temporarily stores a second focus position at the time of detection made
by the first detection system. A controller controls focusing on the
surface of the workpiece on the basis of the calculated deviation, the
stored second focus position and a third focus position when the area on
the workpiece corresponding to the detection area of the first detection
system is positioned in the field of the objective optical system by
relative movement of the workpiece and the objective optical system.
Inventors:
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Suwa; Kyoichi (Tokyo, JP)
|
Assignee:
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Nikon Precision Inc. (Belmont, CA)
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Appl. No.:
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287702 |
Filed:
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April 6, 1999 |
Current U.S. Class: |
250/548; 355/55; 356/399 |
Intern'l Class: |
G01J 001/20; G01N 021/86 |
Field of Search: |
250/548,557,201.2,201.4
355/53,54,55,56
356/399,400,401
|
References Cited
U.S. Patent Documents
4346164 | Aug., 1982 | Tabarelli et al. | 430/311.
|
4391494 | Jul., 1983 | Hershel | 350/442.
|
4650983 | Mar., 1987 | Suwa | 250/204.
|
4747678 | May., 1988 | Shafer et al. | 350/505.
|
5194893 | Mar., 1993 | Nishi | 355/53.
|
5298939 | Mar., 1994 | Swanson et al. | 355/53.
|
5448322 | Sep., 1995 | Sakakibara et al. | 355/53.
|
5483056 | Jan., 1996 | Imai | 250/548.
|
5610683 | Mar., 1997 | Takahashi | 355/53.
|
5646413 | Jul., 1997 | Nishi | 250/548.
|
5650840 | Jul., 1997 | Taniguchi | 250/548.
|
5715039 | Feb., 1998 | Fukuda et al. | 355/53.
|
5742067 | Apr., 1998 | Imai | 250/548.
|
5751428 | May., 1998 | Kataoka et al. | 356/401.
|
5825043 | Oct., 1998 | Suwa | 250/548.
|
Foreign Patent Documents |
61-209831 | Sep., 1986 | JP.
| |
6-204115 | Jul., 1994 | JP.
| |
7-86136 | Mar., 1995 | JP.
| |
7-86137 | Mar., 1995 | JP.
| |
7-220998 | Aug., 1995 | JP.
| |
8-162401 | Jun., 1996 | JP.
| |
Other References
Burn J. Lin, "The Paths To Subhalf-Micrometer Optical Lithography", SPIE
vol. 922, Optical/Laser Microlithography (1988), pp. 256-269.
J. Buckley et al., "Step and scan: A systems overview of a new lithography
tool", SPIE vol. 1088, Optical/Laser Microlithography II, 1989, pp.
424-433.
|
Primary Examiner: Lee; John R.
Assistant Examiner: Pyo; Kevin
Attorney, Agent or Firm: Skjerven Morrill MacPherson LLP, Klivans; Norman R., Schmidt; Mark E.
Parent Case Text
This application is a continuation of Ser. No. 09/134,778 filed Aug. 11,
1998, now abandoned, which in turn is a divisional application of Ser. No.
08/727,695, filed Oct. 7, 1996, now U.S. Pat. No. 5,825,043.
Claims
What is claimed is:
1. A processing apparatus comprising:
a direction system which directs a processing beam to a principal surface
of a workpiece, at least part of the direction system being in a path of
the processing beam;
a first detection system which detects a position of a first area of the
principal surface in a Z-direction substantially perpendicular to the
principal surface of the workpiece, wherein the first area is located
outside a second area of the principal surface of the workpiece to be
processed, and wherein the first detection system detects the position of
the first area of the principal surface in the Z-direction before the
processing beam is directed to the second area;
a second detection system which detects the position of the first area of
the principal surface in the Z-direction when the processing beam is
directed to the second area; and
an adjusting system which adjusts a focus condition of the processing beam
on the workpiece based on the position of the first area in the
Z-direction detected by the first detection system and the position of the
first area in the Z-direction detected by the second detection system.
2. A processing apparatus according to claim 1, wherein the processing
apparatus includes an inspection apparatus which inspects the workpiece.
3. A processing apparatus according to claim 1, wherein the processing
apparatus includes an exposure apparatus in which a pattern is transferred
onto the workpiece.
4. A processing apparatus according to claim 3, wherein the directing
system includes a projection system which projects an image of the pattern
onto the workpiece.
5. A processing apparatus according to claim 3, wherein the processing beam
and the workpiece are relatively moved in a scanning direction during
exposure of the workpiece.
6. A processing apparatus according to claim 1, further comprising:
an auxiliary plate surrounding the workpiece at a height approximately
equal to the principal surface of the workpiece.
7. A processing apparatus according to claim 6, wherein the first detection
system and the second detection system are arranged to detect a position
of a surface of the auxiliary plate in a direction substantially
perpendicular to the surface of the plate.
8. A processing apparatus according to claim 1, further comprising:
a third detection system which detects a position of the second area of the
principal surface of the workpiece in the Z-direction when the position of
the first area of the principal surface in the Z-direction is detected by
the first detection system.
9. A processing apparatus according to claim 1, wherein the processing beam
includes a electron beam.
10. A processing method in which a processing beam is directed onto a
principal surface of a workpiece, the method comprising:
detecting a position of a first area of the principal surface in a
Z-direction substantially perpendicular to the principal surface of the
workpiece, wherein the first area is located outside a second area to be
processed by the beam on the workpiece, the detecting occurring before the
processing beam is applied to the second area;
detecting the position of the first area of the principal surface of the
workpiece in the Z-direction when the processing beam is applied to the
second area on the workpiece; and
adjusting a focus condition of the processing beam on the workpiece based
on the position of the first area of the principal surface in the
Z-direction detected by detecting before the processing beam is applied to
the second area and on the position of the first area of the principal
surface in the Z-direction detected by detecting when the processing beam
is applied to the second area.
11. A processing method according to claim 10, wherein the processing
method includes transferring a pattern onto the workpiece.
12. A processing method according to claim 11, wherein the processing beam
and the workpiece are relatively moved in a scanning direction during
exposure.
13. A processing method according to claim 10, further comprising:
detecting a position of the second area of the principal surface of the
workpiece in the Z-direction.
14. A processing method according to claim 10, wherein the processing beam
includes an electron beam.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor fabrication and more
particularly to a lithography exposure apparatus (aligner) for
transferring a circuit pattern from a mask or a reticle onto a sensitive
substrate.
The present invention also relates to a system for detecting a focal point
on a workpiece (wafer, substrate or plate etc.) and for detecting a tilt
of the workpiece, which is applicable to certain kinds of apparatus such
as an apparatus for manufacturing a workpiece or imaging a desired pattern
in a surface of a workpiece using a laser or electron beam and an
apparatus for optically inspecting the state of a surface of a workpiece.
2. Description of the Related Art
Recently, dynamic random access memory semiconductor chips (DRAMs) having
an integration density of 64 Mbits have been mass-produced by
semiconductor fabrication techniques. Such chips are manufactured by
exposing a semiconductor wafer to images of circuit patterns to form e.g.
ten or more layers of circuit patterns in a superposition manner.
Presently, lithography apparatuses used for such chip fabrication are
projection aligners in which a circuit pattern drawn in a chromium layer
on a reticle (mask plate) is transferred onto a resist layer on a wafer
surface through a 1/4 or 1/5 reduction optical imaging system by
irradiating the reticle with i-line radiation (wavelength: 365 nm) of a
mercury discharge lamp or pulse light having a wavelength of 248 nm from a
KrF excimer laser.
Projection exposure apparatuses (projection aligners) used for this purpose
are generally grouped, according to the types of imaging optical system,
into those using a step-and-repeat system, i.e., so-called steppers, and
those using a step-and-scan system which has attracted attention in recent
years.
In the step-and-repeat system, a process is repeated in which, each time a
wafer is moved to a certain extent in a stepping manner, a pattern image
on a reticle is projected on a part of the wafer by using a reduction
projection lens system formed only of a refractive optical material (lens
element) and having a circular image field or an unit magnification
projection lens system formed of a refractive optical material (lens
element), a prism mirror and a concave mirror and having a noncircular
image field to expose a shot area on the wafer or plate to the pattern
image.
In the step-and-scan system, a wafer is exposed to an image of a portion of
a circuit pattern on a reticle (for example, in the form of a circular-arc
slit) which is projected on the wafer through a projection optical system.
Simultaneously, the reticle and the wafer are continuously moved at
constant speeds at a speed ratio according to the projection
magnification, thus exposing one shot area on the wafer to the image of
the entire circuit pattern on the reticle in a scanning manner.
For example, as described on pp 256 to 269 of SPIE Vol. 922 Optical/Laser
Microlithography (1988), the step-and-scan system is arranged so that,
after one shot area on the wafer has been scanned and exposed, the wafer
is moved one step for exposure of an adjacent shot area, and so that the
effective image field of the projection optical system is limited to a
circular-arc slit. Also, the projection optical system is considered to be
a combination of a plurality of refractive optical elements and a
plurality of reflecting optical elements, such as one disclosed in U.S.
Pat. No. 4,747,678 (to Shafer).
U.S. Pat. No. 5,194,839 (to Nishi) discloses an example of an aligner in
which a step-and-scan system is realized by mounting a stepper reduction
projection lens having a circular image field. This publication also
discloses a method in which a pattern image projected at the time of
scanning exposure is transferred onto a wafer by increasing the depth of
focus (DOF) by a predetermined amount on the wafer.
In the field of lithography technology, it is now desirable to be able to
fabricate semiconductor memory chips having an integration density and
fineness of the 1 or 4 Gbit class by light exposure. Since light exposure
techniques have a long technological history and are based on a large
amount of accumulated know-how, it is convenient to continue use of light
exposure techniques. It is also advantageous to use light exposure
techniques considering drawbacks of alternative electron beam exposure or
X-ray technologies.
It is believed that resolutions in terms of minimum line width (feature
width) of about 0.18 .mu.m and 0.13 .mu.m are required with respect to 1
Gbit and 4 Gbit memory chips, respectively. To achieve resolution of such
a line width, far ultraviolet rays having a wavelength of 200 nm or
shorter, e.g., those produced by an ArF excimer laser, are used for
illumination for irradiating the reticle pattern.
As optical vitreous materials having a suitable transmittance with respect
to far ultraviolet rays (having a wave-length of 400 nm or shorter),
quartz (S.sub.i O.sub.2), fluorite CaF.sub.s, lithium fluoride (L.sub.i
F.sub.2), magnesium fluoride (MgF.sub.2) and so on are generally known.
Quartz and fluorite are optical vitreous materials indispensable for
forming a projection optical system having high resolution in the range of
far ultraviolet rays.
However, it is necessary to consider the fact that, if the numerical
aperture (NA) of a projection optical system is increased to attain high
resolution while the field size is increased, the diameter of lens
elements made of quartz or fluorite becomes so large that it is difficult
to manufacture such lens elements.
Also, if the numerical aperture (NA) of the projection optical system is
increased, the depth of focus (DOF) .DELTA.F is inevitably reduced. In
general, the depth of focus .DELTA.F is defined by wavelength, numerical
aperture NA, a process coefficient Kf (0<Kf<1) as shown below if the
Rayleigh's theory of imaging formation is applied:
.DELTA.F=Kf.multidot.(.lambda./NA.sup.2)
Accordingly, the depth of focus .DELTA.F in the atmosphere (air) is about
0.240 .mu.m if the wavelength is 193 nm, that is, equal to that of ArF
excimer laser light, the numerical aperture NA is set to about 0.75 and
the process coefficient Kf is 0.7. In this case, the theoretical
resolution (minimum line width) .DELTA.R is expressed by the following
equation using process coefficient Kr (0<Kf<1):
.DELTA.R=Kr.multidot.(.lambda./NA)
Accordingly, under the above-mentioned conditions, the resolution .DELTA.R
is about 0.154 .mu.m if the process coefficient Kr is 0.6.
As described above, while it is necessary to increase the numerical
aperture of the projection optical system in order to improve the
resolution, it is important to notice that the depth of focus decreases
abruptly if the numerical aperture is increased. If the depth of focus is
small, there is a need to improve the accuracy, reproducibility and
stability with which an automatic focusing system for coincidence between
the best imaging plane of the projection optical system and the resist
layer surface on the wafer is controlled.
On the other hand, considering the projection optical system from the
standpoint of design and manufacturing, a configuration is possible in
which the numerical aperture is increased without increasing the field
size. However, if the numerical aperture is set to a substantially larger
value, the diameter of lens elements is so large that it is difficult to
form and work the optical vitreous material (e.g. quartz and fluorite).
Then, as a means for improving the resolution without largely increasing
the numerical aperture of the projection optical system, an immersion
projection method may be used in which the space between the wafer and the
projection optical system is filled with a liquid, see U.S. Pat. No.
4,346,164 (to Tabarelli).
In this immersion projection method, the air space between the wafer and
the optical element constituting the projection optical system on the
projection end side (image plane side) is filled with a liquid having a
refractive index close to the refractive index of the photoresist layer,
to increase the effective numerical aperture of the projection optical
system seen from the wafer side, i.e. improving the resolution. This
immersion projection method is expected to attain good imaging performance
by selecting the liquid used.
Projection aligners as presently known generally are provided with an
automatic focusing (AF) system for precisely controlling the relative
positions of the wafer and the projection optical system so that the wafer
surface coincides with the optimum imaging plane (reticle conjugate plane)
of the projection optical system. This AF system includes a surface
position detection sensor for detecting a change in the height position
(Z-direction position) of the wafer surface in a non-contact manner, and a
Z-adjustment mechanism for adjusting the spacing between the projection
optical system and the wafer on the basis of the detected change.
Also in projection aligners presently used an optical type or air
micrometer type sensor is used as the surface position detection sensor,
and a holder (and a Z-stage) for supporting the wafer, provided as the
Z-adjustment mechanism, is moved vertically to sub-micron accuracy.
If such an AF system is provided in an aligner to which the immersion
projection method is applied, it is natural that an air micrometer type
sensor cannot be used and an optical sensor is exclusively used since the
wafer is held in a liquid. In such a case, an optical focus sensor, such
as one disclosed in U.S. Pat. No. 4,650,983 (to Suwa), for example, is
constructed so that a measuring beam (an imaging beam of a slit image) is
obliquely projected into the projection field on the wafer and so that the
beam reflected by the wafer surface is received by a photoelectric
detector through a light receiving slit. The change in the height position
of the wafer surface, i.e., the amount of focus error, is detected from a
change in the position of the reflected beam occurring at the light
receiving slit.
If an oblique incident light type focus sensor such as the one disclosed in
U.S. Pat. No. 4,650,983 is directly mounted in a projection aligner in
which the conventional projection optical system having a working distance
of 10 to 20 mm is immersed in a liquid, a problem described below arises.
In such a case, it is necessary to set in the liquid the optical system of
the projected beam emitted from a projecting objective lens of the focus
sensor to reach the projection field of the projection optical system on
the wafer and the optical system of the reflected beam reflected by the
wafer to reach a light receiving objective lens.
Therefore, the beam of the focus sensor travels through a long distance in
the liquid, so that unless the temperature distribution in the liquid is
stabilized with high accuracy, the projected beam and the received beam
fluctuate by a change in refractive index due to a temperature
nonuniformity, resulting in deterioration in the accuracy of focus
detection (detection of the height position of the wafer surface).
Moreover, to achieve a resolution of 0.15 .lambda.m or less by the
immersion projection method, it is necessary to set the working distance
of the projection optical system to a sufficiently small value, as
mentioned above. Therefore, oblique projection itself of the projected
beam of the oblique incident light type focus sensor from the space
between the projection optical system and the wafer toward the projection
area on the wafer becomes difficult to perform. For this reason, one
important question arises as to how an automatic focusing system
applicable to the immersion projection method is arranged.
On the other hand, aligners (exposure apparatus) having an unit
magnification type (hereinafter described as "1.times.") projection
optical systems are being used in the field of manufacturing liquid
crystal display devices (flat panel displays) as well as in the field of
manufacturing semiconductor devices. Recently, for this kind of aligner, a
system has been proposed in which a plurality of 1.times. projection
optical systems of a certain type are arranged and in which a mask and a
photosensitive plate are moved integrally with each other for scanning. It
is desirable that, ideally, the working distance of the 1.times.
projection optical systems used is extremely small. Each 1.times.
projection optical system is of a single Dyson type such as that disclosed
in U.S. Pat. No. 4,391,494 (to Hershel) or a double Dyson type such as
that disclosed in U.S. Pat. No. 5,298,939 (to Swanson et al.).
In an aligner having such a Dyson type projection optical system, the
working distance (spacing between the exit surface of a prism mirror and
the image plane) can be sufficiently reduced to limit various aberrations
or distortions of the projected image to such small values that there is
practically no problem due to the aberrations or distortions. In this kind
of aligner, therefore, a detection area on the photosensitive substrate of
focus detection by the focus sensor (e.g., the irradiation position of the
projected beam in the oblique incident light system or the air-exhaust
position in the air micrometer system) is ordinarily set to a position
deviating from the effective projection field region of the projection
optical system, that is, set in an off-axis manner.
For this reason, it is impossible to actually detect whether the area of
the substrate exposed to projected light from a circuit pattern is
precisely adjusted in a best focus state or condition.
Also in apparatuses for writing a pattern on a substrate or to perform
processing (or manufacturing) by using a spot of a laser beam or an
electron beam, it is possible that the working distance between the
substrate and the objective lens system (or an electronic lens system) for
projecting the beam becomes so small that an AF sensor capable of
detecting a focusing error of the processing position or the drawing
position on the substrate surface in the field of the objective optical
system cannot be mounted.
In such a case, the detection point of the AF sensor is only placed outside
the field of the objective lens system to detect a focusing error, and it
does not detect whether a focusing error occurs actually at the processing
position or writing position in the field of the objective lens system.
The same can also be said with respect to an apparatus for optically
inspecting a pattern drawn on a reticle or mask for photolithography or a
fine pattern formed on a wafer. That is, this is because this kind of
inspection apparatus is also provided with an objective lens system for
inspection and because the end of the objective lens system faces a
surface of an specimen (a plate) to be inspected while being spaced apart
from same by a predetermined working distance.
Thus, if an objective lens system having a comparatively large magnifying
power and high resolution is used, the working distance is so small that
the same problem relating to the disposition of the AF sensor is
encountered.
SUMMARY OF THE INVENTION
In view of the above-described problems of the related art, the present
invention provides a projection aligner (exposure apparatus) and an
exposure method which enable high-precision focusing control and
high-precision tilt control even if a projection optical system to reduce
the working distance in comparison with the conventional projection
optical system is incorporated.
The invention is directed to a step-and-repeat aligner in which a surface
of a sensitive substrate is exposed to a pattern image projected through
an imaging system or a scanning exposure apparatus (scanning aligner) in
which a mask (or a reticle) and a sensitive substrate are moved relative
to an imaging system while a pattern image is being projected, and to a
system suitable for detecting a focal point and a tilt in these kinds of
exposure apparatus (aligners).
In the present exposure apparatus and method, focusing control and tilt
control are performed with respect to a shot area at a peripheral position
on a sensitive substrate.
The present scanning exposure apparatus and scanning exposure method enable
high-precision focusing control and high-precision tilt control with
respect to an exposed area of a sensitive substrate, without setting a
focus detection area in the projection field of a projection optical
system.
The present focus sensor and focus detection method stably detect an error
in focusing or tilting of a surface of a sensitive substrate immersed in a
liquid in an immersion type projection aligner or scanning aligner
designed to improve the depth of focus. The present focus sensor and focus
detection method are suitable for a manufacturing (processing) apparatus,
a drawing apparatus or an inspection apparatus having an objective optical
system of a small working distance.
The present invention is applicable to a scanning exposure apparatus having
an imaging system (a projection lens system) for projecting an image of a
pattern of a mask (a reticle) on a substrate (a wafer) through an imaging
field, a scanning mechanism (a reticle stage or wafer XY stage) for moving
the mask and the substrate in a scanning direction relative to the imaging
system, and a Z-drive system (a Z stage and Z-actuators) for driving the
substrate and the imaging system relative to each other in a Z-direction
to focus the projected image, or to a projection aligner (i.e, stepper)
having an imaging system for projecting an image of a pattern of a mask on
a substrate through a projection field, a movable stage mechanism which
moves in X and Y directions in order to position the substrate with
respect to the image of the pattern to be projected, and a Z-drive
mechanism for driving the substrate and the imaging system relative to
each other in a Z-direction to focus the image to be projected.
The scanning mechanism or the movable stage mechanism of the exposure
apparatus or aligner may be a mechanism for horizontally maintaining a
mask or substrate. Alternatively, it may be a mechanism for maintaining a
mask or substrate at a certain angle from a horizontal plane, for example,
a vertical stage mechanism for moving a mask or substrate in a horizontal
or vertical direction while maintaining the mask or substrate in a
vertical attitude. In this case, a plane along which the mask or substrate
is moved corresponds to X- and Y-directions, and Z-direction,
perpendicular to each of X- and Y-directions, is also referred to (for
example, in correspondence with the direction of the optical axis of a
laterally-arranged projection optical system or the direction of principal
rays).
According to the present invention, the aligner is provided with a first
detection system having a detection area at a first position located
outside the imaging field of the imaging system and spaced apart from same
in the scanning direction (Y-direction), the first detection system
detecting the position of an obverse (upper) surface of the substrate in
the Z-direction, a second detection system having a detection area at a
second position located outside the imaging field of the imaging system
and spaced apart from the first position in a direction (X) the scanning
direction (Y), the second detection system detecting the position of the
obverse surface of the substrate in the Z-direction, a third detection
system having a detection area at a third position located outside the
imaging field of the imaging system, spaced apart from the same in a
direction (X) perpendicular to the scanning direction (Y) and also spaced
apart from the second position in the scanning direction (Y), and the
third detection system detecting the position of the obverse surface of
the substrate in the Z-direction.
According to the present invention, the aligner is further provided with a
calculator for calculating a deviation between the first Z-position
detected by the first detection system and a target Z-position, and for
temporarily storing the second Z-position detected by the second detection
system at the time of detection made by the first detection system, and a
controller for controlling the Z-drive system on the basis of the
calculated deviation the stored second Z-position and the third Z-position
detected by the third detection system when the area on the substrate
corresponding to the detection area of the first detection system is
positioned in the imaging field of the imaging system by a movement caused
by the scanning mechanism or the movable stage mechanism.
The present invention is applicable to a scanning exposure method in which
all of a pattern of a mask (a reticle) is transferred onto a sensitive
substrate (a wafer) by projecting a part of the mask pattern on the
sensitive substrate through a projection optical system and by
simultaneously moving the mask and the sensitive substrate relative to a
projection field of the projection optical system.
The present method includes the steps of mounting the sensitive substrate
on a holder having an auxiliary plate portion formed so as to surround the
sensitive substrate at a height substantially equal to the height of an
obverse surface of the sensitive substrate, previously reading a focus
error of an exposure area on the sensitive substrate on which area a part
of the pattern of the mask is to be projected, the focus error of the
exposure area being read before the exposure area reaches the projection
field of the projection optical system during scanning movement of the
holder and the sensitive substrate, detecting a focus error of the obverse
surface of a part of the sensitive substrate or the auxiliary plate
portion by an exposure position focus detection system disposed apart from
the projection field of the projection optical system in a direction (X)
perpendicular to the direction (Y) of the scanning movement when the
exposure area on the sensitive substrate reaches the projection field,
adjusting the distance between the projection optical system and the
sensitive substrate on the basis of the detected focus errors so that the
focus error of the exposure area on the sensitive substrate is corrected
in the projection field of the projection optical system.
A focus detection sensor or a focus detection method suitable for
manufacturing (processing) apparatuses, imaging apparatuses and inspection
apparatuses is achieved similarly by replacing the projecting optical
system used for the above-described exposure apparatus (aligner) or the
exposure method with an objective optical system for manufacturing,
writing, imaging or inspection.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing a scanning projection exposure apparatus
(aligner) in a first embodiment of the present invention;
FIG. 2 is a schematic perspective view explaining a scanning exposure
sequence;
FIG. 3 is a schematic perspective view of the disposition of a focus
detection system provided in the vicinity of an end of the projection lens
system shown in FIG. 1;
FIG. 4 is a circuit block diagram of a circuit arrangement in the AF
control unit shown in FIG. 1;
FIG. 5 is a plan view of the positional relationship between a projection
field and detection areas of focus sensors on the wafer in the apparatus
shown in FIG. 1;
FIGS. 6A, 6B, 6C, and 6D are diagrams of the focusing and tilting operation
of the apparatus shown in FIG. 1;
FIG. 7A is a plan view of a layout of detection areas of a focus/tilt
detection system in a second embodiment of the present invention;
FIG. 7B is a side view of a layout of a modified example of the focus/tilt
detection system shown in FIG. 7A;
FIG. 8A is a schematic diagram in a third embodiment of the present
invention in which the invention is applied to a scanning exposure
apparatus (scanning aligner);
FIG. 8B is a perspective view of a vertical carriage applied to the
scanning aligner shown in FIG. 8A;
FIG. 8C is a perspective view of a projection optical system and a focus
detection system provided in the projection aligner shown in FIG. 8A;
FIG. 9 is a cross-sectional view in a fourth embodiment of the present
invention in the construction of which the invention is applied to an
immersion projection exposure apparatus;
FIG. 10 is a diagram showing an example of an optical path layout of a
focus/tilt detection system suitable for the immersion projection exposure
apparatus;
FIGS. 11A and 11B are cross-sectional views of modified examples of the
wafer holder;
FIG. 12A is a diagram showing an example of a manufacturing or imaging or
writing apparatus to which the focus detection sensor of the present
invention is applied;
FIG. 12B is a plan view showing an exemplary layout of the focus detection
system applied to the apparatus shown in FIG. 12A; and
FIG. 13 is a diagram schematically showing the construction of an exemplary
inspection apparatus to which the focus/tilt detection system of the
present invention is applied.
DETAILED DESCRIPTION
FIG. 1 shows the entire construction of a projection exposure apparatus in
a first embodiment of the present invention, and which is a lens-scan type
projection aligner in which a circuit pattern on a reticle is projected
onto a semiconductor wafer through a reduction projection lens system
having circular image fields telecentrically formed on the object side and
the image side while the reticle and the wafer are being moved relative to
the projection lens system to be scanned.
An illumination system shown in FIG. 1 includes an ArF excimer laser light
source for emitting pulse light having a wavelength of 193 nm, a beam
expander for shaping a cross section of the pulse light from the light
source into a predetermined shape, an optical integrator such as a
fly's-eye lens for forming a secondary light source image (a set of a
plurality of point light sources) by receiving the shaped pulse light, a
condenser lens system for condensing the pulse light from the secondary
light source image into pulse illumination light having a uniform
illuminance distribution, a reticle blind (illumination field stop) for
shaping the pulse illumination light into a rectangular shape elongated in
a direction perpendicular to the scanning direction at the time of
scanning exposure, and a relay optical system for imaging the rectangular
opening of the reticle blind on a reticle R in cooperation with a mirror
11 and a condenser lens system 12 shown in FIG. 1.
The reticle R is supported on a reticle stage 14 by vacuum suction
attraction. The reticle stage 14 can move at a constant speed in one
dimension with a large stroke during scanning exposure. The reticle stage
14 is guided on a column structure 13 of an aligner body laterally as
viewed in FIG. 1 to move for scanning. The reticle stage 14 is also guided
so as to move in a direction perpendicular to the plane of the figure.
The coordinate position and the fine rotational deviation of the reticle
stage 14 in an XY-plane are successively measured by a laser
interferometer system (IFM) 17 which projects a laser beam onto a moving
mirror (plane mirror or corner mirror) 16 attached to a portion of the
reticle stage 14 and which receives the beam reflected by the mirror 16. A
reticle stage controller 20 controls motors 15 (such as a linear motor or
a voice coil) for driving the reticle stage 14 on the basis of the
XY-coordinate position measured by the interferometer system 17, thereby
controlling the scanning movement and the stepping movement of the reticle
stage 14.
When a part of a circuit pattern area on the reticle R is irradiated with
rectangular shaped pulse of light emitted from the condenser lens system
12, an imaging light beam from the pattern in the illuminated part is
projected and imaged on a sensitive resist layer applied on the upper
(principal) surface of a wafer W through a 1/4 reduction projection lens
system PL. The optical axis AX of the projection lens system PL is placed
so as to extend through center points of the circular image fields and to
be coaxial with the optical axes of the illumination system 10 and the
condenser lens system 12.
The projection lens system PL includes a plurality of lens elements made
e.g. of two different materials, such as quartz and fluorite having high
transmittance with respect to ultra-violet rays having a wavelength of 193
nm. Fluorite is used mainly to form lens elements having a positive power.
The air in the lens barrel in which the lens elements of the projection
lens system PL are fixed is replaced with nitrogen gas so as to avoid
absorption of the pulse illumination light having a wavelength of 193 nm
by oxygen. Similar nitrogen gas replacement is performed with respect to
the optical path from the interior of the illumination system 10 to the
condenser optical system 12.
The wafer W is held on a wafer holder (chuck) WH which attracts the reverse
(backside) surface of the wafer by vacuum suction. An annular auxiliary
plate portion HRS is provided on a peripheral portion of the wafer holder
WH so as to surround the circumference of the wafer W. The height of the
surface of the auxiliary plate portion HRS is so as to be substantially
flush with the upper surface of the wafer W attracted to the upper surface
of the holder WH. This auxiliary plate portion HRS is used as an
alternative focus detection surface if a detection point of a focus sensor
is positioned outside the contour edge of the wafer W when a shot area at
a peripheral position on the wafer W is scanned and exposed, as described
below in detail.
Further, the auxiliary plate portion HRS can also serve as a flat reference
plate (fiducial plate) for calibration of a system offset of the focus
sensor in the same manner as disclosed in U.S. Pat. No. 4,650,983 (to
Suwa) mentioned above. Needless to say, a special reference plate may be
separately provided for calibration of the focus sensor.
The wafer holder WH is mounted on a ZL stage 30 which can translate in the
Z-direction along the optical axis AX of the projection lens PL, and which
can move in a direction perpendicular to the optical axis AX while tilting
with respect to an XY-plane. The ZL stage 30 is mounted on an XY stage 34
through three Z-actuators 32A, 32B, and 32C. The XY stage 34 is movable
two dimensionally in X- and Y-directions on a base. Each of the
Z-actuators 32A, 32B, and 32C is e.g. a piezoelectric expansion element, a
voice coil motor, or a combination of a DC motor and a lift cam mechanism.
If the three Z-actuators (or Z-drive motors) are each driven in the
Z-direction to the same amount, the ZL stage 30 moves translationally in
the Z-direction (focusing direction) while being maintained parallel to
the XY stage 34. If the three Z-actuators are each driven in the
Z-direction different amounts, an amount and a direction of the tilting of
the ZL stage 30 is thereby adjusted.
The two-dimensional movement of the XY stage 34 is caused by several drive
motors 36 which are e.g. a DC motor for rotating a feed screw or a linear
motor or the like capable of producing a driving force in a non-contact
manner. The drive motors 36 are controlled by a wafer stage controller 35
which is supplied with a measuring coordinate position from a laser
interferometer (IFM) 33 for measuring changes in the position of a
reflecting surface of a moving mirror 31 in the X- and Y-directions.
For example, the entire construction of the XY stage 34 using a linear
motor as drive motor 36 may be as disclosed in Japanese Laid-Open Patent
Application No.(Sho) 61-209831 (Tateishi Electronics Co.) laid open on
Sep. 18, 1986.
With respect to this embodiment, it is assumed here that the working
distance of the projection lens PL is so small that a projected beam of an
oblique incident light type focus sensor cannot be led to the wafer
surface through the space between the surface of the optical element of
the projection lens system PL closest to the image plane and the upper
surface of the wafer W. In this embodiment, therefore, three focus
detection systems GDL, GDC, and GDR of an off-axis type (having a focus
detection point out of the projection field of the projection lens PL) are
disposed around a lower end portion of the barrel of the projection lens
PL.
Of these focus detection systems, the detection systems GDL and GDR are set
so as to have focus detection points positioned on the front and rear
sides of the projection field with respect to the direction of scanning
movement of the wafer W at the time of scanning exposure. When one shot
area on the wafer W is scanned and exposed, one of the detection systems
GDL ad GDR selected according to the direction of scanning movement (plus
direction or minus direction) is operated so as to previously read the
change in the surface height position in the shot area before exposure of
the wafer to the rectangular projected image.
Accordingly, the focus detection systems GDL and GDR function, for example,
as the same pre-read sensors as those of a focus detection system
disclosed in U.S. Pat. No. 5,448,332 (to Sakakibara et al.). In this
embodiment, however, a focus adjustment (or tilt adjustment) sequence
different from that of U.S. Pat. No. 5,448,332 is used and a special focus
detection system is therefore added to the focus detection systems GDL and
GDR. This arrangement is described below in more detail.
The focus detection system GDC shown in FIG. 1 has a detection point in a
non-scanning direction perpendicular to the scanning direction of the
projection field of the projection lens PL as seen on the surface of the
wafer W (i.e., in an XY plane) in accordance with the off-axis method.
However, the focus detection system GDC has another detection point on the
back side of the projection lens PL as viewed in FIG. 1 in addition to its
detection point on the front side.
The focus detection method in accordance with the present invention is
characterized in that the off-axis focus detection system GDC and one of
the pre-reading focus detection systems GDL and GDR are operated in
cooperation with each other. Details of these focus detection systems are
described below.
Information on the height position of a portion of the wafer surface
detected by each of the above-described focus detection systems GDL, GDR,
and GDC (e.g., an error signal or the like representing the amount of
deviation from the best focus position) is input to an automatic focusing
(AF) control unit 38. The AF control unit 38 determines an optimal amount
of driving of each of the Z-drive motors 32A, 32B, and 32C on the basis of
the detection information supplied from the detection systems, and drives
the Z-drive motors 32A, 32b, and 32C to perform focusing and tilt
adjustment with respect to the area of the wafer W on which the projected
image is to be actually imaged.
For this control, each of the focus detection systems GDL and GDR is a
multi-point focus sensor having detection points at a plurality of
positions (e.g., at least two positions) in the rectangular projection
area on the wafer W formed by the projection lens PL, and the AF control
unit 38 is capable of tilt adjustment of the wafer W at least in the
non-scanning direction (X-direction) as well as focusing.
The aligner shown in FIG. 1 is arranged to perform scanning exposure by
moving the XY stage 34 at a constant speed in the Y-direction. The
relation of the scanning movement and the stepping movement of the reticle
R and the wafer W during scanning exposure will now be described with
reference to FIG. 2.
Referring to FIG. 2, a fore-group lens system LGa and a rear-group lens
system LGb represent the projection lens system PL shown in FIG. 1, and an
exit pupil Ep exists between the fore-group lens system LGa and the
rear-group lens system LGb. On the reticle R shown in FIG. 2, a circuit
pattern area Pa having a diagonal length larger than the diameter of the
circular image field on the object side of the projection lens PL is
formed in a frame defined by a shield band SB.
To the image of the area Pa of the reticle R, a corresponding shot area SAa
on the wafer W is exposed in a scanning manner by moving the reticle R at
a constant speed Vr in the minus direction along the Y-axis while moving
the wafer W at a constant speed Vw in the plus direction along the Y-axis,
for example. At this time, the shape of pulse illumination light IA for
illuminating the reticle R is set in the form of a parallel strip or a
rectangle elongated in the X-direction in the area Pa of the reticle, as
shown in FIG. 2. The ends of the shape of pulse illumination light IA
opposite from each other in the X-direction are positioned on the shield
band SB.
A partial pattern contained in the rectangular area in the area Pa of the
reticle R irradiated with the pulse illumination light IA is imaged as an
image SI at the corresponding position in the shot area SAa on the wafer W
by the projection lens system PL (lens systems LGa and LGb). When the
relative scanning of the pattern area Pa on the reticle R and the shot
area SAa on the wafer W is completed, the wafer W is moved one step, for
example, to a certain distance in the Y-direction such that the scanning
start position is set with respect to a shot area SAb adjacent to the shot
area SAa. During this stepping movement, the illumination with pulse
illumination light IA is stopped.
Next, in order to expose the shot area SAb on the wafer W to the image of
the pattern in the area Pa of the reticle R in a scanning manner, the
reticle R is moved at the constant speed Vr in the plus direction of the
Y-axis relative to pulse illumination light IA and the wafer W is
simultaneously moved at the constant speed Vw in the minus direction of
the Y-axis relative to the projected image SI. The speed ratio Vw/Vr is
set to the reduction ratio 1/4 of the projection lens system PL. In
accordance with the above-described schedule, a plurality of shot areas on
the wafer W are exposed to the image of the circuit pattern area Pa of the
reticle R.
The projection aligner shown in FIGS. 1 and 2 can be used as a
step-and-repeat aligner in such a manner that, if the diagonal length of
the circuit pattern area on the reticle R is smaller than the diameter of
the circuit image field of the projection lens system PL, the shape and
size of the opening of the reticle blind in the illumination system 10 are
changed so that the shape of illumination light IA conforms to the circuit
pattern area. In such a case, the reticle stage 14 and the XY stage 34 are
maintained in a relatively-stationary state during exposure of each of
shot areas on the wafer W.
However, if the wafer W moves slightly during exposure, the slight movement
of the wafer W may be measured by the laser interferometer system 33 and
the reticle stage 14 may be slightly moved under control so that the
corresponding small error in the position of the wafer W relative to the
projection lens system PL is canceled by follow-up correction on the
reticle R side. For example, systems for such reticle follow-up correction
are disclosed in Japanese Laid-Open Patent Application Nos. (Hei) 6-204115
and (Hei) 7-220998. Techniques disclosed in these publications may be used
according to one's need.
If the shape or size of the opening of the reticle blind is changed, a zoom
lens system may be provided to enable the pulse light reaching the reticle
blind from the light source to be concentrated within the range matching
with the adjusted opening according to the change in the shape or size of
the opening.
Since the area of the projected image SI is set in the form of a strip or a
rectangle elongated in the X-direction as clearly seen in FIG. 2, tilt
adjustment during scanning exposure may be effected only along the
direction of rotation about the Y-axis, that is, the rolling direction
with respect to the scanning exposure direction in this embodiment.
Needles to say, if the width of the projected image SI area in the
scanning direction is so large that there is a need to consider the
influence of flatness of the wafer surface with respect to the scanning
direction, tilt adjustment in the pitching direction is performed during
scanning exposure. This operation will be described in more detail with
respect to another embodiment of the invention.
The focus detection systems GDL, GDR, and GDC shown in FIG. 1 are disposed
as illustrated in FIG. 3, for example. FIG. 3 is a perspective view
showing the disposition of detection points of the focus detection systems
on the plane on which the circular image field CP of the projection lens
PL on the image side is formed. FIG. 3 shows only the disposition of the
focus detection systems GDL and GDC. The focus detection system GDR is
omitted since it has the same configuration as the detection system GDL.
Referring to FIG. 3, the focus detection system GDC has two detectors GDC1
and GDC2 which are set so that detection points (detection areas) FC1 and
FC2 are positioned on an extension line LLc of the axis of the strip-like
of rectangular projected image SI extending in the circular field CP of
the projection lens PL in a diametrical direction (X-direction). These
detectors GDC1 and GDC2 detect the height position of the upper surface of
the wafer W (or auxiliary plate portion HRS) or a positioning error amount
in the Z-direction with respect to the best focus plane position.
On the other hand, the focus detection system GDL includes in the
embodiment five detectors GDA1, GDA2, GDB1, GBD2, and GDB3 having
respective detection points (detection areas) FA1, FA2, FB1, FB2, and FB3
positioned on a straight line LLa parallel to the extension line LLc. Each
of these five detectors independently detects the height position of a
point on the upper surface of the wafer W (or auxiliary plate portion HRS)
or a positioning error amount in the Z-direction with respect to the best
focus plane position.
The extension line LLc and the straight line LLa are set at a certain
distance from each other in the scanning direction (Y-direction). Also,
the detection point FA1 of the detector GDA1 and the detection point FC1
of the detector GDC1 are set at substantially the same coordinate
positions in the X-direction while the detection point FA2 of the detector
GDA2 and the detection point FC2 of the detector GDC2 are set at
substantially the same coordinate positions in the X-direction.
The detection points FB1, FB2, and FB3 of three detectors GDB1, GDB2, and
GDB3 are disposed so as to cover the area of the strip-like or rectangular
projected image SI in the X-direction. That is, the detection point FB2 is
disposed at a X-coordinate position corresponding to the center (the point
at which the optical axis AX passes) of the area of the projected image SI
in the X-direction while the detection points FB1 and FB3 are disposed at
X-coordinate positions corresponding to positions in the vicinity of the
opposite ends of the projected image SI area in the X-direction.
Therefore, the three detection points FB1, FB2, and FB3 are used for focus
error pre-reading of the surface portion of the wafer W corresponding to
the projected image SI area.
The focus detection system GDR, not shown in FIG. 3, also has three
pre-reading detectors GDE1, GDE2, GDE3 and other two detectors FDD1 and
GDD2 disposed opposite sides of these pre-reading detectors in the
X-direction. For ease of explanation, with respect to this embodiment, it
is assumed that the planes recognized as best focus positions by the
twelve detectors GDA1, GDA2; GDB1, GDB2, GDB3; GDC1, GDC2; GDD1, GDD2;
GDE1, GDE2, GDE3 are adjusted to one XY-plane. That is, no system offset
is provided between the twelve detectors and it is assumed that the
surface height positions of the wafer W detected at the twelve detection
points FA1, FA2; FB1, FB2, FB3; FC1, FC2; FD1, FD2; FE1, FE2, FE3 as
positions at which the detected focus error becomes zero coincide closely
with each other.
For the above-described twelve focus detectors, optical sensors, air
micrometer type sensors, electrostatic capacity type gap sensors or the
like can be used if the end of the projection lens PL is not immersed in a
liquid. However, if an immersion projection system is formed, it is, of
course, impossible to use air micrometer type sensors.
FIG. 4 is a block diagram of an example of the AF control unit 38 for
processing detection signals (error signals) from the focus detection
systems GDL, GDR, and GDC shown in FIGS. 1 and 3. As shown in FIG. 4, one
of the group of detection signals from the five detectors GDA1, GDA2,
GDB1, GDB2, and GDB3 of the pre-reading focus detection systems GDL and
the group of detection signals from the five detectors GDD1, GDD2, GDE1,
GDE2, and GDE3 of the focus detection systems GDR are selected by a
changeover circuit 50 to be supplied to subsequent processing circuits.
The changeover circuit 50 selects the signals from one of the focus
detection systems GDL and GDR in response to a changeover signal SS1
(representing a direction discrimination result) supplied from a position
monitor circuit 52 which discriminates one scanning movement direction of
the wafer stage 34 from the other on the basis of stage control
information from the wafer stage controller 35, and which monitors changes
in the moved position of the wafer W from the pre-read position to the
exposure position. In the state shown in FIG. 4, the changeover circuit 50
is selecting the five detection signals from the focus detection system
GDL.
The detection signals from the pre-reading detectors GDB1, GDB2, and GDB3
with respect to the exposure area (projected image SI) are supplied to a
first calculator 54 for calculating a focus error amount and a tilt error
amount. The calculator 54 supplies a second calculation and memory circuit
56 with error data DT1 and DT2 on focus error amount .DELTA.Zf and tilt
error amount .DELTA.Tx (fine inclination about the Y-axis) of the surface
area of the wafer W previously read at the three detection points FB1,
FB2, and FB3.
On the other hand, the detectors GDA1 and GDA2 supplies the second
calculation and memory circuit 56 with information ZA1 and information ZA2
representing the surface height positions (or focus deviations) at the
detection points FA1 and FA2 simultaneously with the detection of the
wafer surface by the three detectors GDB1, GBD2, and GDB3.
The second calculation and memory circuit 56 calculates, on the basis of
error data DT1, DT2, information ZA1, ZA2 and the relative positional
relationship between the detectors, target values .DELTA.Z1 and .DELTA.Z2
of the height position of the wafer W which should be detected at the
detection points FC1 and FC2 of the detectors GDC1 and GDC2 set at the
projection exposure position with respect to the Y-direction (scanning
direction). The second calculation and memory circuit 56 temporarily
stores the calculated target values .DELTA.Z1 and .DELTA.Z2.
The meaning of the target values .DELTA.Z1 and .DELTA.Z2 is that, if
information ZC1 and information ZC2 detected by the detectors GDC1 and
GDC2 when the surface portions of the wafer W (or auxiliary plate portion
HRS) previously read at the pre-reading detection points FA1 and FA2 reach
the detection points FC1 and FC2 corresponding to the exposure position
are equal to the target values .DELTA.Z1 and .DELTA.Z2, respectively, both
the focus error amount .DELTA.Zf and tilt error amount .DELTA.Tx
determined by pre-reading become zero at the exposure position.
Further, the second calculation and memory circuit 56 outputs the stored
target values .DELTA.Z1 and .DELTA.Z2 to a third calculation and drive
circuit 58 immediately before the pre-read area on the wafer with respect
to the Y-direction arrives at the exposure position at which the projected
image SI is exposed.
Accordingly, in synchronization with a signal SS2 output from the position
monitor circuit 52, the second calculation and memory circuit 56 outputs
signals representing target values .DELTA.Z1 and .DELTA.Z2 temporarily
stored to the third calculation and drive circuit 58 after delaying the
signals by an amount of time determined by the distance between the
straight line LLa and the extension line LLc in the Y-direction and the
speed of movement of the wafer W.
If signal SS2 is output each time the wafer W is moved to be scanned
through a distance corresponding to the width of the projected image SI in
the scanning direction, a certain number of sets of target values
.DELTA.Z1 and .DELTA.Z2 (e.g., five sets) corresponding to a number
obtained by dividing the distance between the straight line LLa and the
extension line LLc in the Y-direction (e.g., about 40 mm) shown in FIG. 3
by the width of the projected image SI (e.g., about 8 mm) are temporarily
stored in the second calculation and memory circuit 56. Accordingly, the
second calculation and memory circuit 56 functions as a memory for storing
target values .DELTA.Z1 and .DELTA.Z2 in a first in-first out (FIFO)
manner.
The third calculation and drive circuit 58 reads, in response to a signal
SS3 from the position monitor circuit 52, detection information ZC1 and
ZC2 on the height position of the surface of the wafer W (or auxiliary
plate portion HRS) detected by the detectors GDC1 and GDC2 immediately
before the area on the wafer W detected at the pre-read position reaches
the exposure position (the position of the projected image SI).
Simultaneously, the third calculation and drive circuit 58 reads the data
of target values .DELTA.Z1 and .DELTA.Z2 (corresponding to the exposure
position) output from the second calculation and memory circuit 56,
determines, by calculation, drive amounts (amounts of position adjustment
or amounts of speed adjustment) corresponding to the Z-drive motors 32A,
32B, and 32C shown in FIG. 1 on the basis of information ZC1 and ZC2 and
target values .DELTA.Z1 and .DELTA.Z2, and outputs determined drive amount
data to the Z-drive motors 32A, 32B, and 32C.
It is to be understood that most of the element of FIG. 4 may be embodied
in a programmed microcontroller or microprocessor, executing a suitable
program which could be written by one of ordinary skill in the art in
light of FIG. 4.
FIG. 5 is a plan view explaining the function of the auxiliary plate
portion HRS formed at the peripheral portion of the wafer holder WH as
shown in FIG. 1. In this embodiment, since all the detection points of the
focus detection systems are positioned outside the projection field CP of
the projection lens PL as described above, there is a possibility of some
of the focus detection points being located outside the perimeter of wafer
W at the time of scanning exposure of some of a plurality of shot areas
SAn on the wafer arranged at the peripheral portion of the wafer W.
For example, as shown in FIG. 5, when a peripheral shot area SA1 of the
wafer W positioned on the holder WH by using a prealignment notch NT is
scanned and exposed, the end focus detection point FA1 (or FD1) of the
pre-reading focus detection system GDL (or GDR) and the detection point
FC1 of the exposure position focus detection system GDC are located
outside the wafer W. In this state, it is difficult to normally perform
focusing and tilt adjustment.
A main function of the auxiliary plate portion HRS is enabling normal
focusing and tiling in such a situation. As shown in FIG. 5, the detection
point FA1 (or FD1) and the detection point FC1 located outside the of the
wafer W are set so as to be positioned on the surface of the auxiliary
plate portion HRS. Accordingly, it is desirable that the height of the
surface of the auxiliary plate portion HRS is substantially equal to that
of the surface of the wafer W.
More specifically, the surface of the wafer W and the surface of the
auxiliary plate portion HRS are made flush with each other within the
detection ranges which correspond to the detection points FA1 (FA2), FC1
(FC2), and FD1 (FD2) and in which the desired linearity of the focus
detectors corresponding to the detection points are ensured. Further,
since the surface of the auxiliary plate portion HRS is used as an
alternative to the surface of the wafer W, its reflectivity is set on the
same order or to the same value as the reflectivity of a standard
(silicon) wafer. For example, a mirror-finished surface is preferred as
the auxiliary plate portion HRS.
If the wafer W (on wafer holder WH) is moved to be scanned in the direction
of the arrow shown in FIG. 5, the detection points FA1, FA2; FB1, FB2, FB3
of the focus detection system GDL are selected as pre-reading sensors with
respect to the shot area SA1. In this event, if the distance between the
extension line LLc corresponding to the center of the projected image SI
in the Y-direction and the straight line LLa on which the detection points
of the focus detection system GDL are disposed is DLa and if the distance
between the extension line LLc and the straight line LLb on which the
detection points of the other focus detection system GDR are disposed is
DLb, DLa and DLb are set so that DLa is approximately equal to DLb in this
embodiment. From the speed Vw of the wafer W at the time of scanning
exposure, the delay time .DELTA.t taken for the focus pre-read position on
the wafer W to reach the exposure position is .DELTA.t=DLa/Vw (sec.).
Accordingly, the time for temporary storage of target value data .DELTA.Z1
and .DELTA.Z2 in the second calculation and memory circuit 56 shown in
FIG. 4 is substantially equal to the time lag .DELTA.t.
However, the distance DLa and the distance DLb may be selected so that DLa
does not equal DLb according to a restriction relating the construction of
the aligner. Needless to say, in such a case, the delay time of supply of
the target values .DELTA.Z1 and .DELTA.Z2 are set to different lengths
with respect to use of the pre-reading focus detection system GDL and use
of the focus detection system GDR.
The focusing and tilting operations of the first embodiment arranged as
described above is now described with reference to FIGS. 6A through 6D.
FIG. 6A schematically shows a state of the upper surfaces of the wafer W
and the auxiliary plate portion HRS detected by the pre-reading focus
detection system GDL at an instant during scanning exposure of the
peripheral shot area SA1 of the wafer W as shown in FIG. 5.
In FIGS. 6A through 6D, a horizontal line BFP represents the optimum focus
plane of the projection lens PL. The detector GDB1 that detects the
position of the wafer surface in the Z-direction at the focus detection
point FB1 in the shot area SA1 outputs a detection signal representing
.DELTA.ZB1 as a Z-position error (amount of defocusing) of the wafer
surface with respect to the plane BFP. Similarly, the detectors GDB2 and
GDB3 that detect errors of the position of the wafer surface in the
Z-direction at the focus detection points FB2 and FB3 output detection
signals representing errors .DELTA.ZB2 and .DELTA.ZB3. Each of these
Z-position errors has a negative value if the wafer surface is lower than
the best focus plane BFP, or has a positive value if the wafer surface is
higher than the best focus plane BFP.
The values of these errors .DELTA.ZB1, .DELTA.ZB2, and .DELTA.ZB3 are input
to the first calculation and memory circuit 54 shown in FIG. 4. The first
calculation and memory circuit 54 determines parameters of an equation
representing an approximate plane APP (actually an approximate straight
line) shown in FIG. 6B of the entirety of the pre-read portion in the shot
area SA1 by the method of least squares or the like on the basis of these
error values. The parameters thereby determined are focus error amount
.DELTA.Zf and tilt error amount .DELTA.Tx of the approximate plane APP, as
shown in FIG. 6B. The values of error amount .DELTA.Zf and amount
.DELTA.Tx thus calculated are output as data DT1 and DT2 to the second
calculation and memory circuit 56. In this embodiment, the focus error
amount .DELTA.Zf is calculated as an error substantially at the middle
point (corresponding to detection point FB2) of the shot area SA1 in the
X-direction.
When the detectors GDB1, GDB2, and GDB3 detect Z-position errors as
described above, the detectors GDA1 and GDA2 simultaneously detect
Z-position errors .DELTA.ZA1 and .DELTA.ZA2 of the wafer surface or the
surface of the auxiliary plate portion HRS with respect to the best focus
plane at the detection points FA1 and FA2. These errors .DELTA.ZA1,
.DELTA.ZA2 are temporarily stored in the second calculation and memory
circuit 56.
Immediately after this detection and storage, assuming that the approximate
plane APP such as that shown in FIG. 6B is corrected so as to coincide
with the best focus plane BFP as shown in FIG. 6C, that is, the wafer
holder WH is adjusted in the Z-direction and the tilting direction so that
.DELTA.Zf=0 and .DELTA.Tx=0, the second calculation and memory circuit 56
calculates the Z-position target value .DELTA.Z1 to be detected at the
detection point FA1 and the Z-position target value .DELTA.Z2 to be
detected at the detection point FA2 on the basis of data DT1 and DT2
(error amount .DELTA.Zf and .DELTA.Tx), Z-position errors .DELTA.ZA1,
.DELTA.ZA2 actually measured at the detection points FA1 and FA2 and the
distance DS between the middle point of the shot area and each of the
detection points FA1 and FA2 in the X-direction. The calculated Z-position
target values .DELTA.Z1 and .DELTA.Z2 are temporarily stored in the second
calculation and memory circuit 56 until the pre-read area on the wafer W
reaches the area of the projected image SI (exposure position).
When the pre-read area on the wafer W reaches the exposure position, the
third calculation and drive circuit 58 shown in FIG. 4 reads the detection
signals from the focus detectors GDC1 and GDC2 for detecting Z-position
errors at the detection points FC1 and FC2. If, for example, the pre-read
area on the wafer W is in a state such as shown in FIG. 6D immediately
before it reaches the exposure position, the detector GDC1 outputs
detection signal ZC1 representing a Z-position error at the detection
point FC1 while the detector GDC2 outputs detection signal ZC2
representing a Z-position error at the detection point FC2.
Then the third calculation and drive circuit 58 calculates the drive
amounts for the three Z-actuators 32A, 32B, and 32C necessary for tilting
the wafer holder WH and/or translating the wafer holder WH in the
Z-direction so that the values of detection signals ZC1 and ZC2 supplied
from the detectors GDC1 and GDC2 become respectively equal to the
Z-position target values .DELTA.Z1 and .DELTA.Z2 which are supplied from
the second calculation and memory circuit 56 by being delayed. The third
calculation and drive circuit 58 supplies the Z-actuators 32A, 32B, 32C
with signals corresponding to the calculated drive amounts.
The shot area SA1 of the upper surface of wafer W is thereby precisely
adjusted to coincide with the best focus plane BFP at the exposure
position. As a result, the projected image SI of the pattern of the
reticle R to be maintained in an optimal imaged state is exposed in the
scanning mode of the shot area.
For this operation in the first embodiment, each detector in the
pre-reading focus detection system GDL and each detector in the exposure
position focus detection system GDC are set (calibrated) so as to output a
detection signal indicating that there is no focus error when the surfaces
of the wafer W or the auxiliary plate portion HRS coincide with the best
focus plane BFP. However, it is difficult to strictly set the detectors in
such a state. In particular, a detection offset between the detectors GDA1
and GDA2 (GDD1 and GDD2) in the pre-reading focus detection system GDL
(GDR) and the exposure position focus detectors GDC1 and GDC2 steadily
defocuses the pattern image formed on the wafer W for exposure.
Therefore, an offset value between the height position in the Z-direction
at which the detector GDC1 detects the zero focus error and the height
position in the Z-direction at which the detector GDA1 (GDD1) detects the
zero focus error may be measured and stored by simultaneously performing
focus detection by these detectors on the extremely high flatness surface
of a reflective glass plate (or fiducial plate) provided on the wafer
holder WH. This surface may be structure HRS or another structure separate
from structure HRS. As a result, the correction by the stored offset value
may be made when the Z-actuators 32A, 32B, and 32C are drive on the basis
of the Z-position errors detected by the exposure position focus detectors
GDC1 and GDC2.
The construction of a focus and tilt sensor in accordance with a second
embodiment of the present invention is next described with reference to
FIGS. 7A and 7B. With respect to the second embodiment, a situation is
supposed in which the projected image SI contained in the circular field
of the projection lens PL has a comparatively large maximum width in the
Y-direction (scanning direction) such that the influence of a tilt of the
surface of wafer W in the Y-direction, i.e., pitching, is considerable.
As shown in FIG. 7A, an exposure position focus detector GDC1 (not
illustrated) is provided which has two detection points FC1a and FC1b
disposed symmetrically about extension line LLc in the Y-direction above
the projected image SI, and another exposure position focus detector GDC2
(not illustrated) is provided which has two detection points FC2a and FC2b
disposed symmetrically about extension line LLc in the Y-direction below
the projected image SI. Further, a pre-reading focus detector GDA1 having
two detection points FA1a and FA1b disposed symmetrically about straight
line LLa in the Y-direction and a pre-reading focus detector GDA2 (not
illustrated) having two detection points FA2a and FA2b disposed
symmetrically about straight line LLa in the Y-direction are provided.
Similarly, a pre-reading focus detector GDD1 (not illustrated) having two
detection points FD1a and FD1b disposed symmetrically about straight line
LLb in the Y-direction and a pre-reading focus detector GDD2 having two
detection points FD2a and FD2b disposed symmetrically about straight line
LLb in the Y-direction are provided.
Pre-reading focus detectors GDBn (n=1, 2, 3) (not illustrated) having pairs
of detection points FB1a, FB1b; FB2a, FB2b; FB3a, FB3b, and pre-reading
focus detectors GDEn (n=1, 2, 3) (not illustrated) having pairs of
detection points FE1a, FE1b; FE2a, FE2b; FE3a, FE3b are also provided.
Each pair of detection points are spaced apart from each other in the
Y-direction.
The focus detection system shown in FIG. 7A reproduces adjustment amounts
(i.e., target values .DELTA.Z1 and .DELTA.Z2) necessary for correcting the
pre-read surface configuration (i.e., error amount .DELTA.Zf and
.DELTA.Tx) of each shot area at the detection points of the off-axis
detectors GDC1 and GDC2 in the same manner as the above-described first
embodiment, thereby enabling focus adjustment in the Z-direction and tilt
adjustment in the X-direction (rolling direction) of the exposure area.
In this embodiment, since the pre-reading focus detection system GDL (GDR)
and the exposure position focus detection system GDC have pairs of
detection points (FAna and FAnb; Fbna and FBnb; FCna and FCnb; FDna and
FDnb; FEna and FEnb) spaced apart by a certain distance in the
Y-direction, a tilt error amount .DELTA.Ty of the pre-read shot area in
the pitching direction can be detected from the differences between
Z-position errors at the detection points ( . . . na, . . . nb) forming
pairs in the Y-direction, and adjustment amounts (i.e., target values
.DELTA.ZA1, .DELTA.ZA2) necessary for correcting the surface configuration
of the shot area including of the tilt error amount .DELTA.Ty, can be
reproduced at the detection points (FCna and FCnb) of the off-axis
detectors GDC1 and GDC2.
The detectors GDB1, GDB2, and GDB3 for detecting the focus positions at the
detection positions FB1, FB2, and FB3 shown in FIG. 3 are disposed as
systems independent of each other by being fixed to a lower portion of the
projection lens PL. However, at least these three detectors GDB1, GDB2,
and GDB3 may be arranged to detect the focus positions at the detection
points FB1, FB2, and FB3 through a common objective lens system. The same
can also be said with respect to the group of three detectors GDE1, GDE2,
and GDE3 for detecting the focus positions at the detection points FE1,
FE2, and FE3 shown in FIG. 5.
Further, a common objective lens system may be used for the same purpose
with respect to the group of six detectors for detecting the focus
positions at the six detection points Fbna and FBnb (n=1, 2, 3) shown in
FIG. 7A or the other group of six detectors for detecting the focus
positions at the six detection points FEna and FEnb (n=1, 2, 3). An
arrangement of using a common objective lens system for detectors for
detecting the focus positions at a plurality of detection points is
therefore described briefly with reference to FIG. 7B.
FIG. 7B is a schematic side view of the positional relationship between the
projection lens and the detectors corresponding to the six detection
points FBna and FBnb (n=1, 2, 3) and the four detection points FA1a, FA1b,
FA2a, and FA2b shown in FIG. 7A as seen in the Y-direction in FIG. 7A.
Accordingly, the scanning direction of the wafer W in FIG. 7B is a
direction perpendicular to the plane of the figure and the five detection
points FA1a, FBna (n=1, 2, 3), and FA2a arranged in a row in the X
direction at the leftmost position in FIG. 7A are representatively shown
in FIG. 7B. Another row of detection points FA1b, FBnb (n=1, 2, 3), and
FA2b are adjacent to the five detection points FA1a, FBna (n=1, 2, 3), and
FA2a (in a direction perpendicular to paper of FIG. 7B). In this
embodiment, the focus positions at these ten points are detected through
the objective lens system.
As shown in FIG. 7B, illumination light ILF from an illumination optical
system 80A including a light source (e.g. a light emitting diode, a laser
diode, a halogen lamp or the like) capable of emitting light in a
wavelength range to which the resist layer on wafer W is not sensitive is
emitted through each of ten small slits formed in a multi-slit plate 81A.
The ten small slits are disposed in correspondence with the ten detection
points FBna, FBnb (n=1, 2, 3), FA1a, FA1b, FA2a, and FA2b set on the wafer
W. Light transmitted through the small slits is incident upon an objective
lens 84A of a projection system via a lens system 82A and a reflecting
mirror 83A and is deflected by a prism 85A by a desired angle to form a
slit image at each detection point.
The illumination optical system 80A, the multi-slit plate 81A, the lens
system 82A, the reflecting mirror 83A, the objective lens 84A and the
prism 85A constitute a projection system of an oblique incident light type
focus detection unit. The solid lines in the optical path from the
multi-slit plate 81A to the wafer W shown in FIG. 7B represent principal
rays of transmitted light from the small slits, and the broken lines in
the optical path represent typical imaging rays Slf of the small slit
imaging light imaged at the detection point FB2a (or FB2b).
The reflected light of the small slit imaging light reflected at each
detection point on the wafer W is again imaged on a receiving slit plate
81B via a prism 85B, an objective lens 84B, a reflecting mirror 83B and a
lens system 82B disposed generally symmetrically with respect to the
projection system. Ten small receiving slits disposed in correspondence
with the small slits in the projection multi-slit plate 81A are formed in
the receiving slit plate 81B. Light transmitted through these receiving
slits is received by a light receiving device 80B which is a plurality of
photoelectric detection elements.
As the photoelectric detection elements of the light receiving device 80B,
ten photoelectric detection elements are provided in correspondence with
the positions of the small slits of the receiving slit plate 81B to
separately detect the focus positions at the detection points on the
wafer. The light receiving device 80B, the receiving slit plate 81B, the
lens system 82B, the reflecting mirror 83B, the objective lens 84B and the
prism 85B constitute a light receiving system of the oblique incident
light type focus detection unit. The solid lines in the optical path from
the wafer W to the receiving slit plate 81B shown in FIG. 7B represent
principal rays of the small slit images normally reflected by the wafer W,
and the broken lines in the optical path represent typical imaging rays
RSf from the detection point FB2a (or FB2b) to the receiving slit plate
81B.
The projection system and the receiving system shown in FIG. 7B are mounted
on an integrally-formed metal member so that the positions of the
components are accurately maintained relative to each other. The metal
member is rigidly fixed on the lens barrel of the projection lens PL.
Another focus detection unit constructed in the same manner is disposed on
the opposite side of the projection lens PL to separately detect the focus
positions at the ten detection points FEna, FEnb (n=1, 2, 3), FD1a, FD2a,
FD1b, and FD2b shown in FIG. 7A.
With respect to the pair of detection points FC1a and FC1b and the pair of
detection points FC2a and FC2b shown in FIG. 7A, oblique incident light
type focus detection units each having a projection system and a receiving
system arranged in the Y-direction of FIG. 7A (direction perpendicular to
paper in FIG. 7B) may be provided on the opposite sides of the projection
lens PL in the X-direction. Also in the case where the focus position
detection points are disposed as shown in FIG. 5, the oblique incident
light type focus detection unit shown in FIG. 7B can also be applied in
the same manner.
A scanning aligner to which the present automatic focusing/tilt control
system is applied is next described in accordance with a third embodiment
of the present invention with reference to FIG. 8A. This embodiment is
applicable to a scanning aligner for a large substrate e.g. 300 mm
diameter or greater having a 1.times. projection optical system formed of
a tandem combination of a first-stage Dyson type (catadioptric) projection
imaging system consisting of a pair of prism mirrors PM1 and PM2, a lens
system PL1 and a concave mirror MR1 and a second-stage Dyson type
projection imaging system consisting of a pair of prism mirrors PM3 and
PM4, a lens system PL2 and a concave mirror MR2. Such an aligner is
disclosed in U.S. Pat. No. 5,298,939 (to Swanson et al.), for example.
In the aligner shown in FIG. 8A, a mask M provided as an original plate and
a plate P provided as a photosensitive substrate are integrally supported
on a carriage 100, and a pattern on the mask M is transferred onto the
plate P as a 1.times. (unit magnification) erect image by moving the
carriage 100 to the left or right as viewed in FIG. 8A relative to the
projection field of the 1.times. projection optical system and
illumination light IL so as to scan the mask M and plate P.
In the case of the projection optical system for this type of aligner, it
is desirable to minimize the spacing between the incidence plane of the
prism mirror PM1 and the surface of the mask M and the spacing between the
exit plane of the prism mirror PM4 and the upper surface of the plate P
for reducing deteriorations in imaging performance (various aberrations
and image distortion). In other words, if these spacings can be
sufficiently reduced, the design of the lens systems PL1 and PL2 disposed
on the optical axes AX1 and AX2 becomes easier. Therefore, to achieve the
desired imaging performance, it is necessary to reduce the spacing between
the prism mirror PM1 and the mask M and the spacing between the prism
mirrors PM4 and the plate P.
In view of this condition, for focusing and tilt adjustment of the pattern
image projected by this projection, prereading focus detection systems GDL
and GDR and an exposure position off-axis type focus detection system GDC
such as those of the first embodiment (FIG. 3) or the second embodiment
(FIGS. 7A, 7B) are provided around the prism mirror PM4 as shown in FIG.
8A to precisely coincide the surface of the plate P and the best focus
plane BFP at the exposure position immediately below the prism mirror PM4,
by slightly moving the plate P in the Z-direction and the tilting
direction.
Further, pre-reading focus detection systems GDL' and GDR' and an exposure
position off-axis type focus detection system GDC' may be disposed around
the prism mirror PM1 on the mask M side so as to face the mask M, as shown
in FIG. 8A. These focus detection systems make it possible to detect a
focus error and a tilt error of the area of the mask M irradiated with
illumination light IL with respect to the prism mirror PM1 and to measure,
in real time, a small deviation in the Z-direction (a focus shift of the
image plane) and a tilt deviation (inclination of the image plane) of the
best focus plane (i.e., a conjugate plane of reticle R) formed at a
predetermined working distance from the prism mirror PM4.
Thus, in the aligner shown in FIG. 8A, the image plane on which the pattern
of the mask M is projected and imaged in an optimal condition by the
projection optical system and the surface of the plate P can be adjusted
to coincide with each other highly accurately during scanning exposure.
The aligner shown in FIG. 8A may be constructed so that mask M and plate P
stand vertically. FIG. 8B is a perspective view of an exemplary structure
of a scanning aligner having a vertical carriage for vertically holding
mask M and plate P and for integrally moving mask M and plate P with
respect to a projection optical system for scanning. A scanning aligner
having mask M and plate P held vertically in this manner is disclosed in
Japanese Laid-Open Patent Application No. (Hei) 8-162401, for example.
Referring to FIG. 8B, the entirety of the vertical type scanning aligner is
constructed on a fixed base 120A which is placed on a floor with vibration
isolators interposed between four corner portions of the fixed base 120A
and the floor. Side frame portions 121A and 121B are provided on opposite
side portions of the fixed base 120A so as to stand vertically (in the
X-direction). A mask M is placed inside the side frame portion 121A while
a plate P is placed inside the side frame portion 121B. In the side frame
portion 121A, there-fore, an opening is formed in which an end portion of
an illumination unit 122 having optical systems for illuminating mask M
with exposure illumination light and for mask-plate alignment is inserted,
as illustrated.
A guide base portion 123 is provided on the fixed base 120A so as to extend
in the scanning direction (Y-direction) between the side frame portions
121A and 121B. Two straight guide rails 123A and 123B are formed on the
guide base portion 123 so as to extend in the Y-direction parallel to each
other. A vertical carriage 125 is supported by fluid bearings or magnetic
floating bearings on the guide rails 123A and 123B reciprocatingly movably
in the Y-direction. The vertical carriage 125 is driven in the Y-direction
in a non-contact manner by two parallel linear motors 124A and 124B having
stators fixed on the guide base portion 123.
The vertical carriage 125 has a mask-side carriage portion 125A vertically
formed inside the side frame portion 121A to hold mask M and a plate-side
carriage portion 125B vertically formed inside the side frame portion 121B
to hold plate P. A mask table 126A for slightly moving mask M in the X- or
Y-direction in an XY-plane or in a rotational (.theta.) direction and for
slightly moving mask M in the Z-direction while holding mask M is provided
on the mask-side carriage portion 125A. On the other hand, a plate stage
126B for slightly moving plate P in the X- or Y-direction in an XY-plane
or in a rotational (.theta.) direction and for slightly moving plate P in
the Z-direction while holding plate P is provided on the plate-side
carriage portion 125B.
A projection optical system PL such as one disclosed in Japanese Laid-Open
Patent Application No. (Hei) 8-162401 mentioned above is used in this
embodiment. The projection optical system PL is constructed by arranging a
plurality of sets (e.g., seven sets) of "1.times." erect type double Dyson
systems in the direction perpendicular to the X-direction. The plurality
of sets of double Dyson systems are integrally combined and housed in a
casing which is generally T-shaped as viewed in an XZ-plane. The
projection optical system PL thus constructed is mounted by being
suspended from upper end portions of the opposite side frame portions 121A
and 121B so that predetermined working distances from mask M and plate P
are maintained.
In the entire casing of the projecting optical system PL, mask M-side focus
detection systems GDC', GDL', and GDR' on the mask M side and plate P-side
focus detection systems GDC, GDL, and GDR are provided so as to face mask
M and plate P, respectively, as shown in FIG. 8A. The detection points
defined by the pre-reading focus detection systems GDL, GDL', GDR, and
GDR' may be set in correspondence with the projection fields of the
plurality of sets of double Dyson systems or may be arranged at
predetermined intervals irrespective of the placement of the projection
fields.
FIG. 8C is a perspective view of an example of a layout of detectors in
mask M-side focus detection systems GDC', GDL', and GDR' provided in the
casing of the projection optical system PL shown in FIG. 8B. The effective
projection fields DF1, DF2, DF3, DF4, DF5, . . . of the plurality of sets
of double Dyson systems are set as trapezoidal areas elongated in the
X-direction perpendicular to the scanning direction. The trapezoidal
projection fields DFn (n=1, 2, 3. . . ) are arranged in such a manner that
the trapezoidal projection fields of each adjacent pair of double Dyson
systems overlap each other by their oblique sides as seen in the
X-direction.
While only the projection fields DFn on the mask M side are illustrated in
FIG. 8C, the projection fields on the plate P side are also arranged in
the same manner. For example, the projection field DF2 shown in FIG. 8C is
defined by a double Dyson system such as that shown in FIG. 8A including
two concave mirrors MR2a and MR2b, and the projection field DF4 is defined
by a double Dyson system including two concave mirrors MR4a and MR4b.
As shown in FIG. 8C, detectors GDA1', GDB1', GDB2' . . . , GDA2' (detectors
GDA2' not being seen in FIG. 8C) for the pre-reading focus detection
system GDL' and detectors GDD1', GDE1', GDE2' . . . , GDD2' (detectors
GDD2' not being seen in FIG. 8C) for the pre-reading focus detection
system GDR' are disposed on the opposite sides (on the front and rear
sides with respect to the scanning direction) of the plurality of
projection fields DFn. Also, exposure position focus detectors GDC1' and
GDC2' (detector GDC2' not being seen in FIG. 8C) are disposed at the
opposite ends of the entire array of the plurality of projection fields
DFn in the X-direction perpendicular to the scanning direction.
Each of the focus detectors described above is e.g. an air micrometer type
electrostatic gap sensor. They may alternatively be oblique incident light
type focus detectors. While only the focus detectors for detection on the
mask M side are illustrated in FIG. 8C, a plurality of detectors are also
arranged in the same manner in the focus detection systems GDC, GDL, and
GDR for detection of the plate P.
Adjustment portions KD1 and KD2 for adjusting various optical
characteristics of the plurality of sets of double Dyson systems are
provided on side portions of the casing of the projection optical system
PL shown in FIG. 8C. Therefore, a mechanism is provided to adjust the
Z-direction position, i.e., to set a mechanical (optical) focus offset
detected as a best focus plane by each focus detector, if the position of
the best focus plane on the mask M side or plate P side is changed in the
Z-direction in FIG. 8C by the optical characteristic adjustment.
This mechanism may be e.g. a mechanism which mechanically adjusts the
position of a focus detector in the Z direction, or a mechanism which
optically adjusts the position recognized as the best focus position by
the focus detector in the Z direction, so that the optical path length is
changed optically. Alternatively, the mask or plate are automatically
adjusted for focussing in the Z direction according to detection signals
which represent a focus error, and an offset is added to its moved
position in the Z direction.
A fourth embodiment in accordance with the present invention is next
described with reference to FIG. 9. This embodiment is applicable to an
apparatus for performing projection exposure while immersing a projection
end portion of a projection lens system PL in a liquid as described above.
FIG. 9 is a cross-sectional view of a portion of the apparatus from the
end of the projection lens system PL and to a wafer holder WH.
A positive lens element LE1 having a flat lower surface Pe and a convex
upper surface is fixed on the end of the projection lens system PL inside
the lens barrel. The lower surface Pe of this lens element LE1 is finished
so as to be flush with the end surface of the extreme end of the lens
barrel, so that a flow of a liquid LQ is disturbed only to a minimal
extent. To a lens barrel end portion of the projection lens system PL
immersed in liquid LQ, detectors of pre-reading focus detection systems
GDL and GDR and an exposure position focus detection system GRD which are
similar to those shown in FIG. 1 are attached so that their extreme end
portions are immersed in liquid LQ.
A plurality of attraction surfaces 113 for attracting the reverse surface
of wafer W by vacuum suction are formed in a central inner bottom portion
of the wafer holder WH. More specifically, the attraction surfaces 113 a
plurality of circular-band-like land portions which have a height of about
1 mm and which are formed concentrically with each other with a
predetermined pitch in the diametrical direction of the wafer W. Each of
the grooves formed in central portions of the circular land portions
communicates with a tubing 112 in the wafer holder WH. The piping 112 is
connected to a vacuum source for vacuum suction.
In this embodiment, the spacing (substantial working distance) between the
lower surface Pe of the lens element LE1 at the end of the projection lens
system PL and the upper surface of the wafer W (or auxiliary plate portion
HRS) in an optimum focus state, i.e., the thickness of liquid LQ in which
a projection optical path is formed, is set to be 5 mm or less.
Accordingly, the depth Hq of liquid LQ filling the wafer holder WH may be
two to several times larger than this thickness (5 mm or less), and the
height of a wall portion LB vertically formed at the peripheral end of the
wafer holder WH is about 10 to 25 mm. Thus in this embodiment, the
thickness of liquid LQ in the imaging optical path corresponding to the
working distance of the projection lens system PL is reduced, so that the
total volume of liquid LQ filling the wafer holder WH is smaller and hence
temperature control of the liquid [LQ] is easier.
In the region of liquid LQ in which the projection optical path is formed,
a part of the illumination energy is absorbed when exposure light passes
therethrough, so that an irradiation heat fluctuation can easily occur. If
the depth Hq of liquid LQ is small, an increase in temperature due to such
irradiation heat fluctuation occurs easily and an adverse effect of
reducing the stability of temperature control may result. In such a case,
a better effect is obtained by setting the depth Hq of liquid LQ to a
value several times the substantial working distance, in order to disperse
the influence of irradiation heat fluctuation in the large-volume liquid
layer.
To provide focus detection systems GDL, GDR, and GDC as an optical type
detection system in an immersion projection system such as that shown in
FIG. 9, one prevents the projected beam obliquely incident upon the
surface of wafer W or auxiliary plate portion HRS and the beam reflected
by this surface from intersecting the interface between liquid LQ and air.
An example of a focus/tilt detection system suitable for such an immersion
projection type aligner is therefore described with reference to FIG. 10.
FIG. 10 shows the construction of a focus detection system GDL disposed in
the vicinity of a projection lens system PL. Other detection systems GDR
and GDC are constructed in the same manner as the detection system GDL. In
FIG. 10, the same components as those shown in FIG. 9 are indicated by the
same reference characters or numerals.
Referring to FIG. 10, a prism mirror 200 formed of a glass block and having
a lower portion immersed in liquid LQ is fixed in the vicinity of a
peripheral portion of the projection lens system PL. The prism mirror 200
has reflecting surfaces 200a and 200b partially immersed in liquid LQ, and
flat surfaces 200c and 200d through which the projected beam or reflected
beam travels out of the glass of the prism mirror 200 into liquid LQ or
out of liquid LQ into the glass. Also the prism mirror 200 has a flat
upper surface.
A multi-slit plate 205 is illuminated through a condenser lens or a
cylindrical lens 203 with light LK (having a non-actinic wavelength
relative the resist on wafer W) from a light source 202 such as a light
emitting diode (LED) or a laser diode (LD) for forming a projected beam
for focus/tilt detection. A plurality of transmission slits corresponding
to detection points (areas) FAn and FBn of the focus detection system GDL
are formed in the slit plate 205. The light from each transmission slit is
reflected by a beam splitter 207 and is incident upon an objective lens
209 to be converged as an imaging beam forming a slit image on the upper
surface of wafer W.
The imaging beam emergent from the objective lens 209 enters the prism
mirror 200 through the upper end surface of the same, is normally
reflected by the reflecting surface 200a, and enters liquid LQ through the
flat surface 200c to be obliquely incident upon the surface of wafer w to
irradiate the same. The beam reflected by wafer W enters the prism mirror
200 through the opposite flat surface 200d, is normally reflected by the
reflecting surface 200b and travels out of the prism mirror 200 through
the upper end surface. This reflected light beam passes through an
objective lens 211 and is reflected by a reflecting mirror 213 disposed at
a pupil position of the objective lens 211.
The beam reflected by the mirror 213 travels reversely through the
objective lens 211 and again travels via the reflecting surface 200 and
the flat surface 200d of the prism mirror 200 to again irradiate wafer W.
The light beam again reflected by wafer W travels via the flat surface
200c and the reflecting surface 200a of the prism mirror 200, passes the
beam splitter 207 and is incident on a photoelectric detector 215. The
photoelectric detector 215 is a plurality of light receiving elements
corresponding to the slits of the slit plate 205. The photoelectric
detector 215 separately outputs detection signals with respect to the
detection points FAn and FBn, respectively.
Thus, the focus/tilt detection system shown in FIG. 10, is arranged as a
double-path system in which the projected beam reflected by wafer W is
again reflected by wafer W, and can therefore have higher sensitivity for
detection of an error in the wafer W surface position in the Z-direction
in comparison with a single-path system.
In this embodiment, a glass block (prism mirror 200) is provided at the
extreme end of the focus/tilt detection system and is positioned so as to
be partially immersed in liquid LQ, so that the projected beam and the
reflected beam do not pass any interface between liquid LQ and air, thus
providing a stable beam path. Moreover, the effective length of the path
in liquid LQ through which the projected beam or reflected beam travels is
reduced by virtue of the prism mirror 200, thereby avoiding any reduction
in accuracy due to temperature variation of liquid LQ at the time of
Z-position measurement.
Modified examples of the structure of the wafer holder WH shown in FIGS. 1
and 5 are described with reference to FIGS. 11A and 11B. FIG. 11A is a
cross-sectional view of a wafer holder WH to be mounted in a projection
exposure apparatus for performing immersion exposure. In this example,
fine Z-drive units 220 such as piezoelectric elements are provided which
can slightly move an auxiliary plate HRS surrounding an attraction surface
113 on which wafer W is supported. The fine Z-drive units 220 move the
auxiliary plate HRS in the Z-direction by a stroke of about several tens
of micro-meters.
If the difference between the height of the surface of wafer W placed on
the attraction surface 113 of the wafer holder WH and the height of
surface of the auxiliary plate HRS in the Z-direction is larger than an
allowable difference, this Z-drive unit 220 is used to correct the height
of surface of the auxiliary plate HRS so that the difference is reduced to
a value smaller than the allowable value.
As mentioned above with reference to FIG. 5, the surface of the auxiliary
plate HRS functions as an alternative detection surface for the focus
detection points FA1 (or FA2), FC1 (or FC2), and FD1 (or FD2) located
outside wafer W when shot area SA1 at the peripheral portion of wafer W is
exposed. However, when inner shot area SA2 (see FIG. 5) of wafer W is
exposed, these focus points are positioned on wafer W. Therefore, the
focus detectors GDA1, GDA2, GDC1, GDC2, GDD1, and GDD2 having detection
points each of which is not exclusively positioned on one of the surface
of the auxiliary plate HRS and the surface of wafer W must accurately
measure the Z-position on each of these surfaces. That is, it is necessary
for the positions in the Z-direction of the surfaces of the auxiliary
plate HRS and wafer W to be within the linear focus measuring range of the
each focus detectors GDAn, GDCn and GDDn.
For example, if the linear focus measuring range of the focus detectors is
.+-.10 micrometers, the Z positional deviations of the surfaces of the
auxiliary plate HRS and wafer W are limited within the range of several
micrometers. However, the thickness of wafers varies in a tolerance
determined by the SEMI standard, and it is difficult to limit the
thicknesses of all usable wafers within the range of several micro-meters.
Therefore, when wafer W is attracted to the wafer holder WH shown in FIG.
11 before exposure, the difference between the Z-position of a suitable
portion of the wafer W surface (e.g., a central portion of a peripheral
shot area) and the Z-position of the surface of the auxiliary plate HRS is
measured by using one of the focus detection systems (GDL, GRD, GDC)
before exposure. If the difference exceeds the allowable range (e.g.,
several micro-meters), the height of the auxiliary plate HRS is adjusted
so that the difference is within the allowable range by controlling the
fine Z-drive units 220 shown in FIG. 11A. Since the wafer holder WH shown
in FIG. 11A is filled with liquid LQ, the fine Z-drive units 220 are
"waterproofed" to prevent the liquid from entering the units.
The construction shown in FIG. 11B is next described. FIG. 11B is a
cross-sectional view of a modified example of the structure including a
wafer holder WH and a ZL stage 30, which is suitable for exposure of a
wafer in air. The components corresponding to those shown in FIG. 11A are
indicated by the same reference characters or numerals. Referring to FIG.
11B, the wafer holder WH is constructed as a chuck on which only an
attraction surface 113 for supporting wafer W is formed, and which is
fixed on a ZL stage 30.
An auxiliary plate HRS is mounted on the ZL stage 30 with fine Z-drive
units 220 interposed therebetween. Each function point PV of three
Z-actuators 32A, 32C, and 32B (32B not being seen in FIG. 11B) for driving
the ZL state 30 in the Z-direction and a tilting direction are set to
points at a peripheral portion of the ZL stage 30 substantially at the
same height as the wafer mount surface (attraction surface 113) of the
wafer holder WH.
Also in the arrangement shown in FIG. 11B, the height of the auxiliary
plate HRS is adjusted to that of the upper surface of wafer W by using
fine Z-drive units 220 in the same manner as shown in FIG. 11A. This
structure of the ZL stage 30 and the Z-actuators 32 shown in FIG. 11B, in
which the height of the height of the function points PV are set to the
same level as the wafer surface, may also be applied to the aligner shown
in FIG. 1. Also, the wafer holder WH of FIG. 11A may be mounted on the ZL
stage 30 of FIG. 11B to form a focusing and tilting stage suitable for
immersion projection exposure apparatus or its method.
The present invention has been described with respect to applications to
exposure apparatus. However, the above-described embodiments can be
modified in various ways with-out departing from the scope of the present
invention. For example, the focus detection systems GDL, GDR, and GDC may
include electrostatic capacity type gap sensors or air micrometer type gap
sensors in the case of an aligner for performing projection exposure in
air. Also, the present invention is applicable e.g. to any of the
step-and-repeat type, step-and-scan type and "1.times." scanning type
projection aligners using, as exposure light, g-line (463 nm) or i-line
(365 nm) from a mercury discharge lamp or pulse light (248 nm) from KrF
excimer laser.
According to the present invention, precise focusing and tilt control at
the exposure position can be realized while the working distance of the
projection optical system mounted in the projection aligner is set to an
extremely small value, so that correction of various aberrations and
distortion correction in optical design of the projection optical system
become easier and the transparent optical element positioned near the
image plane, in particular, can be reduced in size.
Each of the focusing/tilt control systems in accordance with the
above-described embodiments of the present invention is applicable to a
certain type of projection exposure apparatus. However, the present
invention is also applicable to focus/tilt detection systems for beam
processing (manufacturing) apparatuses, writing apparatuses, inspection
apparatuses and the like and is not limited to semiconductor fabrication.
These beam processing apparatuses, writing apparatuses and inspection
apparatuses are provided with an optical or electrooptical objective
system to which the present invention can be applied as a focus detection
system for detecting a focus on a substrate, specimen or workpiece.
FIG. 12A shows the construction of a focus detection system applied to an
objective optical system of an apparatus for processing a workpiece with a
laser or electron beam or for writing a pattern on a workpiece, and FIG.
12B shows a planar layout of detection points of the focus detection
system shown in FIG. 12A.
Referring to FIG. 12A, a processing or writing beam LBW is deflected
unidimentionally or two-dimensionally by a scanning mirror 300 and travels
via a lens system 301, a fixed mirror 302 and a lens system 303 to be
incident upon a beam splitter 304. The beam LBW is reflected by the beam
splitter 304 to be incident upon a high-resolution objective system 305
having a small working distance. The beam LBW is condensed into a small
spot having a predetermined shape (e.g., a variable rectangular shape) on
a workpiece WP by the objective system 305.
The workpiece WP is attracted to and fixed on the same holder WH as that
shown in FIG. 11A or 11B. An auxiliary plate HRS is attached integrally to
the holder WH around the workpiece WP. The holder WH is fixed on an
unillustrated XYZ-stage to be moved two-dimensionally in a horizontal
direction and in a direction perpendicular to paper as viewed in FIG. 12A.
The holder WH is also moved slightly in the vertical direction
(Z-direction) for focusing.
The apparatus shown in FIG. 12A is also provided with an optical fiber 310
for emitting illumination light for observation, alignment or aiming, a
beam splitter 311 and a lens system 312 for leading the illumination light
to the above-mentioned beam splitter 304, and a light receiving device
(e.g. or photomultiplier, image pickup tube, CCD or the like) 314 for
photoelectrically detecting reflected light, scattered and diffracted
light or the like from the workpiece WP obtained through the objective
system 305.
Pre-reading focus detection systems GDL and GDR and a processing position
focus detection system GDC are provided around the objective system 305.
FIG. 12B shows a field 305A of the objective system 305 and a planar
layout of detection points of the focus detection systems disposed around
the field 305A. For convenience, the center of the field 305A is set at
the origin of an XY coordinate system. A rectangular area in the field
305A indicates the range through which the spot of the beam LBW scans by
the deflection of the beam caused by the scanning mirror 300.
Focus detectors GDA1, GDBn, and GDA2 on the left-hand side of the field
305A of the objective system are disposed so that detection points FA1,
FB1, FB2, FB3, and FA2 is set in a row parallel to the Y-axis. Also, focus
detectors GDD1, GDEn, and GDD2 on the right-hand side of the field 305A
are disposed so that detection points FD1, FE1, FE2, FE3, and FD2 is set
in a row parallel to the Y-axis.
On the other hand, a focus detector GDC1 provided above the field 305A is
set so that three detection points FD1a, FD1b, and FD1c are placed on a
line passing the two detection point FA1 and FD1 and parallel to the
X-axis while a focus detector GDC2 provided below the field 305A is set so
that three detection points FD2a, FD2b, and FD2c are placed on a line
passing the two detection point FA2 and FD2 and parallel to the X-axis. In
this embodiment, a set of the focus detectors GDA1, GDBn and GDA2 and a
set of the focus detectors GDD1, GDEn and GDD2 are selected as the focus
pre-reading function while the workpiece WP moves in the X-direction. On
the other hand, the focus pre-reading function is achieved by selecting a
set of the focus detectors GDA1, GDC1 and GDD1 and a set of the focus
detectors GDA2, GDC2 and GDD2 while the workpiece WP moves in the
Y-direction. This embodiment is arranged so that the detection points of
the focus detectors GDBn, GDC1, GDC2, and GDEn can be changed for
detecting a focus of the processing position. For example, when the
workpiece WP is moved in the X direction from the left-hand side to the
right-hand side of FIG. 12, one of three pairs of detection points FD1a
and FD2a, detection points FD1b and FD2b, and detection points FD1c and
FD2c may be selected for focus detection of the processing position while
the detection points FA1, FB1, FB2, FB3, and FA2 are being used for
pre-reading.
This arrangement is intended to achieve an effect described below. That is,
the position of the spot of the processing or drawing light beam LBW
changes in the is scanning range 305B. Therefore, when for example, the
light spot is positioned at the leftmost end of the scanning range 305B as
seen in FIG. 12B, the two detection points FD1a and FD2a are selected for
processing position focus detection. When the light spot is positioned at
the rightmost end of the scanning range 305B, the two detection points
FD1c and FD2c are selected for processing position focus detection.
In this manner, the reproducibility and accuracy of focus control or tilt
control are improved. The holder 12A shown in FIG. 12A is slightly moved
in the focusing (Z) direction and in a tiling directions on the XY stage.
As is a drive system and a control system for this movement, those shown
in FIG. 4 can be used without being substantially modified.
As described above, the focus detection system shown in FIG. 12A and 12B is
arranged to enable pre-reading detection of the focus in each of the
directions of the two-dimensional movement of workpiece WP and to enable
the focus detection point for the processing position to be selected
according to the position of the beam spot in the field 305. As a result,
even a peripheral portion of workpiece WP is precisely processed (imaged)
in an accurately focused state or pattern imaging can be performed thereon
in such a state.
An inspection apparatus to which the focus/tilt detection system of the
present invention can be applied is described briefly with reference to
FIG. 13 which shows an example of an apparatus for optically inspecting
defects in patterns drawn on a mask or reticle for photolithography or
defects in circuit patterns of a semiconductor device or liquid crystal
display device formed on a substrate.
In recent years, techniques for examining the quality of an inspected
pattern formed on a specimen (substrate) and checking the presence or
absence of extraneous materials or particles and damage by enlarging the
inspected pattern through an objective optical system, by forming an
enlarged image of the pattern by a CCD camera or the like and by analyzing
an image signal obtained from such an image have been constructively
introduced into this kind of inspection apparatus.
In such a case, it is important to improve the accuracy with which an
accurately enlarged image of the inspected pattern is obtained. An
objective system having high resolution and a large field size and capable
of forming an image with minimized aberrations and distortion is therefore
required. Such an objective system naturally has a small working distance
and is ordinarily designed as a through the lens (TTL) type such that
focus detection is made through the objective system. However, a TTL
optical focus detection system entails a problem of limiting the detection
sensitivity (the amount of change in detection signal with respect to an
error in focusing a specimen) because of a restriction due to the
numerical aperture (NA) of the objective system.
If a TTL focus detection system is formed so as to use light having a
wavelength different from that of illumination light for inspection,
aberration correction must be taken into consideration with respect to the
wavelength ranges of inspection illumination light and focus detection
illumination light in the optical design of the objective system. In such
a case, the lens cannot always be designed optimally with respect to
inspection illumination light.
Then, as shown in FIG. 13, a plurality of sets of focus detection systems
GDC, GDL, and GDR are provided around an objective lens 330 for inspection
in the same manner as those shown in FIGS. 12A and 12B. A specimen WP to
be inspected is e.g. a mask having a pattern Pa formed on its lower
surface. The specimen WP is supported at its peripheral end on a
frame-like two-dimensionally-movable stage 331 having an opening. The
objective lens 330 is mounted in an upward-facing state on a base member
332 for guiding movement of the stage 331. An enlarged image of a local
area in pattern Pa is imaged on an imaging plane of an image pickup device
336 through a beam splitter 334 and a lens system 35.
On the opposite side of the specimen WP, a condenser lens 338 of an
illumination optical system is disposed coaxial with the axis AX of the
objective lens 330. Illumination light from an optical fiber 340 travels
through a condenser lens 341, an illumination field stop 342 and a lens
system 343 to be incident upon the condenser lens 338, thereby irradiating
the area on the specimen WP corresponding to the field of the objective
330 with a uniform illuminance.
In the above-described arrangement, the focus detection systems GDC, GDL
and GDR are mounted on the base member 332 together with the objective 30
so as to upwardly face the pattern Pa. A plurality of focus detectors (a
plurality of detection points) are provided in the focus detection systems
GDL and GDR provided for pre-reading, while at least one pair of focus
detectors is provided in the focus detection system GDC for detection at
the inspection position.
Also in the focus detection system shown in FIG. 13, the specimen WP on the
stage 331 may be moved vertically along the optical axis AX or tilted on
the basis of focus position information detected by the focus detectors by
using a control circuit such as that shown in FIG. 4. In the inspection
apparatus shown in FIG. 13, however, only an effect of obtaining a
high-quality enlarged image of the pattern Pa imaged by the image pickup
device 36 may suffice. Therefore, a focus adjuster 352A or 352B for
slightly moving the objective lens 330 or the lens system 335 along the
optical axis AX may be provided instead of the means for vertically moving
the specimen WP.
An inspection apparatus in which a mask pattern Pa provided as a specimen
WP is positioned so as to face downward has been described by way of
example with reference to FIG. 13. Needless to say, this embodiment can be
directly applied to an inspection apparatus in which pattern Pa faces
upward, while the objective lens faces downward. In the apparatus shown in
FIG. 13, a transmitted image of pattern Pa is inspected by a coaxial
transmission illumination system.
However, the illumination system may be changed so that coaxial reflection
illumination light is introduced through the beam splitter 334 in the
direction of the arrow 350 in FIG. 13. In such a case, the enlarged image
received by the image pickup device 336 is formed by imaging reflected
light from the pattern Pa.
Further, another method may be used in which a spatial filter with a
transmission portion having a desired shape is removably placed at the
position of a Fourier transform plane formed in the optical path of the
illumination optical system or in the imaging optical system to enable a
bright field image or a dark field image of pattern Pa to be selectively
imaged on the image pickup device 336.
This disclosure is illustrative and not limiting; further modifications
will be apparent to one of ordinary skill in the art in light of this
disclosure, and are intended to fall within the scope of the appended
claims.
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